Nadal has been out of action with tendinitis in his right knee since losing to Robin Soderling in the fourth round of the French Open, where the Spaniard had been the four-time defending champion.
Nadal is hoping to resume playing at next month's Montreal Masters.
"Until I'm 100 percent, you have to be realistic," Nadal said. "What's left of the season is going to be difficult because, after stopping for a month - practically two - you lose your rhythm while others continue on competing."
Nadal was unable to defend his Wimbledon title, and Roger Federer instead won his record 15th Grand Slam trophy and retook the No. 1 ranking.
"Without a doubt, I also have one advantage over Federer - that I'm 23 years old," said Nadal, who is four years younger than Federer.
Nadal is returning from a leg problem for the third time since October.
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